At present, the mainstream products of solar cells are bulk crystalline Si solar cells using a crystalline Si substrate.
FIG. 20 shows a general sectional structure of a bulk crystalline Si solar cell 110 disclosed in Patent Document 1.
An opposite conductivity-type region 104 in which phosphorus (P) atoms are diffused at a high concentration is formed on the light incidence plane side of a p-type bulk region 105 constituting a p-type silicon semiconductor substrate, thereby forming a pn junction between the p-type bulk region and the opposite conductivity-type region. Further, an antireflection film 106 including a silicon nitride film or a silicon oxide film is provided on the side of the light incidence plane.
When light is incident on the light incidence plane, photoproduction carriers are produced in a semiconductor region 103 including the opposite conductivity-type region 104, the p-type bulk region 105, and the p+-type region 107. In order to collect the carriers as a current and lead it to an output terminal, surface electrodes (bus bar electrodes 101 and finger electrodes 102) composed of a metal material such as silver as a main component are provided on the light incidence plane side, and a back collecting electrode 108 composed of aluminum or silver and back output electrodes 109 composed of silver are provided on the opposite side.
These electrodes are generally formed by printing and firing Al paste or Ag paste. In this case, the firing conditions including the paste composition and the firing temperature are determined on the basis of the following indexes:
(1) contact characteristic at an electrode/semiconductor interface (ohmic characteristic);
(2) adhesive strength characteristic at an electrode/semiconductor interface (electrode strength characteristic);
(3) resistivity characteristic of an electrode (series resistance characteristic); and
(4) pn-junction characteristic (junction recombination current (diode current) characteristic and leakage current characteristic).
In particular, when an electric contact is formed by a process (fire-through process) in which electrode forming metal paste is applied directly on the antireflection film 106 and fired, the paste composition is controlled to conform to the method, and the firing conditions are determined in consideration of:
(5) the fire-through property of the antireflection film.
FIG. 21 is a drawing the surface electrodes as viewed from the light incidence plane (surface) side. In the drawing, reference numeral 101 denotes a bus bar electrode, and reference numeral 102 denotes a finger electrode.
The surface electrodes generally include the finger electrodes 102 (branch electrodes) with a narrow line width and the bus bar electrodes 101 (stem electrodes) with a thick line width to which at least one of the ends of each finger electrode 102 is connected.
In particular, in order to increase as much as possible the effective light receiving area, it is necessary that the surface electrodes are as narrow as possible. However, as the line width decreases, the series resistance value of the surface electrodes generally increases (series resistance loss increases), thereby decreasing the fill factor FF of solar cell characteristics. Therefore, Ag paste containing as a main component Ag having the highest conductivity among metal materials is generally used.
Patent Document 2 discloses the results of measurement of conditions for producing a polycrystalline silicon ingot in order to achieve a high efficiency of energy conversion in consideration of the concentration relations between impurities such as C, O, B, and P.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-274356    Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-251010    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-332279